This page provides various back titration question generators. Some allow you to set the difficulty, give you clues and step by step worked solutions. They are suitable for all A-level Chemistry students. Traditionally AQA test back titrations a lot but recently OCR have started to as well.
New guided version
This above does very occasionally give %'s over 100 unlike a real scenario
Inspired by a nasty real y13 AQA question (UK, 2017 A2 paper 1 Q11), maybe try the original question first, as it's a bit easier as they give you the equations. Refresh page to generate a new version